What is the process of a substance changing directly from solid to gas called?

Correct answer: (a) Sublimation

Explanation

A substance changing directly from solid to gas, without becoming a liquid, is sublimation: camphor, naphthalene balls, iodine and dry ice do this.

Evaporation is liquid to gas at the surface, and melting is solid to liquid. The reverse of sublimation, gas straight to solid, is deposition.